Problem 18 - Chemistry The Central Science

Question

(a) What are the units usually used to express the rates of reactions occurring in solution? (b) As the temperature increases, does the reaction rate increase or decrease? (c) As a reaction proceeds, does the instantaneous reaction rate increase or decrease?

Step-by-Step Solution

Answer

(a) Reaction rates in solution are usually expressed in units of moles per liter per unit time (\(\text{M} \cdot \text{s}^{-1}\) or \(\text{M} \cdot \text{min}^{-1}\)). (b) As the temperature increases, the reaction rate typically increases due to greater kinetic energy and more frequent, energetic collisions between particles. (c) As a reaction proceeds, the instantaneous reaction rate generally decreases because of decreasing reactant concentrations and a lower rate of collisions between reacting particles.

Step 1: (a) Units for reaction rates

For reactions occurring in solution, reaction rates are generally expressed in units of moles per liter per unit time (\(\text{M} \cdot \text{s}^{-1}\) or \(\text{M} \cdot \text{min}^{-1}\)), where M represents molarity or moles per liter.

Step 2: (b) Effect of temperature on reaction rate

As the temperature of a reaction increases, the reaction rate typically increases. This is because as temperature increases, the kinetic energy of the reacting particles increases, leading to more frequent and energetic collisions between particles. According to the Arrhenius equation, the rate constant (k) increases with temperature, resulting in a higher reaction rate.

Step 3: (c) Change in instantaneous reaction rate as a reaction proceeds

As a reaction proceeds, the instantaneous reaction rate usually decreases. This is because over time, the concentrations of the reactants decrease, leading to a lower rate of collisions between reacting particles. Consequently, the rate at which the reaction progresses slows down as it proceeds. Note that this general statement holds true for most, but not all, reactions; the specific behavior can vary depending on the reaction mechanism.